Georgia Power Co. has delayed the start of Vogtle Unit 3, the only nuclear unit under consruction in the U.S., after detecting vibrations related to the cooling system piping, the companys said in a Wednesday filing with the SEC.

The vibrations occurred during start-up and pre-operational testing, the Southern Company subsidiary said.

Unit 3 will be placed in service in April, beyond the first quarter it expected, Georgia Power said. The delay will raise capital costs by $15 million per month, it said.

The projected schedule for Unit 3 primarily depends on the progress in start-up, final component and pre-operational testing that may be affected by equipment or other operational failures, the company said.

Georgia Power warned of possible additional schedule delays or cost increases. It cited required engineering changes or remediation related to plant systems, structures or components based on new technology that only recently began initial operation in the global nuclear industry at scale.

“The ultimate outcome of these matters cannot be determined at this time,” it said. Updates will be provided during Southern Company’s earnings call in February, the company said.

The U.S. Nuclear Regulatory Commission in August authorized Southern Nuclear Operating to load fuel and start operation at Vogtle Unit 3, “the first reactor to reach this point in the agency’s combined license process.”

Southern Company’s CEO told investors in November 2021 that the cost of two nuclear reactors at Plant Vogtle increased again due to staffing problems and added maintenance costs. It delayed the start by three months: Unit 3, to the third quarter of 2022, and Unit 4 to be in service in the second quarter of 2023. 

Vogtle was originally scheduled to have its first unit be operational starting in 2016, but technology and operational problems have extended the timeline and pushed up costs.